1. Field of the Invention
The invention concerns superconductive ceramic and particularly concerns superconductive ceramic having the formula YBa.sub.2 Cu.sub.3 O.sub.7 that can be produced as a shaped article such as a continuous fiber or a thin film by a sol-gel process.
2. Description of the Related Art
Beginning in 1986, there were reports of a new class of ceramic oxide materials which exhibit superconductivity at unprecedentedly high temperatures. Among these is orthorhombic YBa.sub.2 Cu.sub.3 O.sub.x (where x is typically between 6.5 and 7.2) which is often called "YBa.sub.2 Cu.sub.3 O.sub.7 " or "123 superconductive ceramic" by virtue of its metallic molar ratios. It is the first material known to exhibit superconductivity above the boiling point of liquid nitrogen (77.degree. K. or -196.degree. C.). The historical development of these ceramic oxide materials is described in Clarke, "The Development of High-Tc Ceramic Superconductors: An Introduction," Advanced Ceramic Materials; , Vol. 2, No. 3B, pp 273-292, (1987).
Other articles in that same special issue of Advanced Ceramic Materials describe preliminary studies of the crystallography, physical properties, processing, and theory of these new ceramic oxide materials. Most of these studies report fabrication of bulk ceramic samples by conventional ceramic processing methods. For example, Johnson et al. (pp 364-371) describe mixing, milling, calcining (900.degree. C.), and regrinding of oxide and carbonate powders to yield YBa.sub.2 Cu.sub.3 O.sub.7 powder that was formed into bulk ceramic samples which were then sintered at 950.degree.-990.degree. C. for 3-8 hours. A few of the studies report liquid chemical methods for preparing these new ceramic oxide materials.
For example, Dunn et al. (pp 343-352) synthesize YBa.sub.2 Cu.sub.3 O.sub.7 powder by the amorphous citrate process, a well known technique for synthesizing perovskite compounds. After dissolving nitrate salts of Y, Ba and Cu, citric acid was added and water was evaporated from the solution until it became a viscous liquid which was further dehydrated to form an amorphous solid precursor that was then fired to form the desired compound. The large volume of fugitive combustion products formed during pyrolysis would convert the dried precursor into a powder during the firing.
Johnson et al. (pp 337-342) form YBa.sub.2 Cu.sub.3 O.sub.7 powder by freeze-drying aqueous solutions of Y, Ba and Cu nitrates.
Cima et al. (pp 329-336) are primarily concerned with improving the microstructure of YBa.sub.2 CuO.sub.7 and report high density samples made from powders produced by solid state reaction. However, at page 332, they state that they "have synthesized Y.sub.1.2 Ba.sub.0.8 u(OH).sub.x by preparing an emulsion of an aqueous solution of Y, Ba, and Cu salts in heptane and coprecipitating the hydroxides in the emulsion droplets (microreactors) by bubbling ammonia through the emulsion."
Dagani, C&E News, May 11, 1987, pages 7-16, also reports studies of YBa.sub.2 Cu.sub.3 O.sub.7 and says (pp 13-14):
"Because these ceramics tend to be brittle and fragile, they cannot be drawn out to form wires as copper and other ductile metals can. . . (W)ires are made by filling hollow metal wire (such as silver) with Y-Ba-Cu-O powder and drawing out the wire to the desired diameter . . . At 77 K, they measure a current density of about 175 amp per sq cm. Jin (of Bell Labs) admits this is `rather low,` but he says it's consistent with typical bulk superconductors. PA0 "Another approach is to mix the ceramic powder or its ingredients with an organic binder and then extrude a thin `noodle` of the plastic material through a die. At that stage, the extruded wire can easily be bent or deformed. But it must be fired to burn off the binder and sinter the powders into a current-carrying filament." PA0 Clarke (pp 280) says concerning YBa.sub.2 Cu.sub.3 O.sub.7 : "The first films to exhibit complete superconductivity above liquid nitrogen were prepared by electron beam evaporation through a partial pressure of oxygen onto sapphire and MgO substrates maintained at typically 450.degree. C. . . . More recently a number of groups have been successful in producing thin films by d.c. magnetron sputtering . . . and also by ion beam sputtering."
At the same page, Clarke discusses fibers, including attempts to pull fibers from a gel and then fire them to convert the gel to superconducting form.
Clarke says (p 277) "that any rare earth element (other than La) could be used to substitute, wholly or in part, for the yttrium without markedly altering the superconducting transition temperature". Dagani says (p 12) researchers have found that "when yttrium is replaced by samarium, europium, gadolinium, dysprosium, holmium, or ytterbium, the ceramic still becomes superconducting around 90 K."